Batteries based on lithium (Li) are attractive due to their high energy density compared to other commercial batteries. Lithium-ion batteries are used commercially today in computers, cell phones, and related devices. Lithium-based batteries (including lithium-ion, lithium-sulfur, and lithium-air systems) have significant potential in transportation applications, such as electric vehicles.
Battery lifetime is often a critical factor in the marketplace, especially for commercial, military, and aerospace applications. Previous methods of extending battery life include employing long-life cathode and anode materials, and restricting battery operation to avoid conditions detrimental to battery life. Examples of such detrimental conditions include high and low temperatures, high depths of discharge, and high rates. These restrictions invariably lead to under-utilization of the battery, thus lowering its effective energy density. In addition, precise control of cell temperature with aggressive thermal management on the pack level is usually required, which adds significantly to system weight, volume, and cost.
Lithium-sulfur batteries have theoretical energy densities of 2500 Wh/kg (watt-hours per kilogram), in contrast to 560 Wh/kg for lithium-ion batteries. Commercialization of lithium-sulfur batteries has been hindered by technical difficulties. When a sulfur electrode is discharged, it forms a series of polysulfides that are soluble in common battery electrolytes. The dissolved compounds can migrate to the lithium electrode, effectively creating an internal short mechanism with greatly reduced energy efficiency. Metal lithium forms dendrites during repeated cycling due to non-uniform dissolution and deposition. These dendrites are highly reactive with electrolytes and can even penetrate the separator to create internal shorting. The impact of this shorting is a reduction of cycle life, energy density and cycling efficiency, as the polysulfides continue to build on the anode, and sulfur is lost from the cathode to the anode.
It has proven difficult to maintain electrical isolation of the anode and cathode, while at the same time, provide lithium-ion conduction that will not limit the power performance of the battery cell. A successful battery separator layer should have a wide electrochemical stability window to be stable against the battery anode and cathode. In addition, the separator layer needs to have limited electronic conductivity in order to prevent electrical leakage between the two electrodes. When both requirements are imposed, the available materials and techniques are very limited.
The formation of lithium dendrites at the anode can also limit the cycle life of a lithium sulfur battery by driving up the cell resistance. If the current density is not uniform across the surface of the lithium anode, lithium can be preferentially deposited in the areas with the highest current density. Deposition in these areas exacerbates the current density non-uniformity which propagates the formation of lithium dendrites on the anode. As the number of dendrites on the surface of the anode increases, the cell resistance increases, limiting the power performance of the cell.
Prior approaches attempting to reduce polysulfide crossover in lithium-sulfur cells include cathode nanostructuring or encapsulation, electrolyte optimization (e.g., salt concentration or solvent composition), electrolyte additives (such as LiNO3) to protect lithium, and dual-phase or multilayer electrolytes. None of these approaches has the potential to completely eliminate self-discharge.
In view of the foregoing shortcomings, new battery cell structures are needed to address important commercialization issues associated with lithium-sulfur batteries. What is needed in particular is a cell configuration that can stop the crossover to, and deposition of, polysulfides formed during discharge on the anode. Improved separators for lithium-sulfur batteries, and methods to make and use those improved separators, are therefore desired.